Biaxially stretched polyester films, such as biaxially stretched polyethylene terephthalate films, have excellent strength, heat resistance, dimensional stability, chemical resistance, aroma retaining properties, and the like, and therefore are useful for various packaging materials. Therefore, it is expected that packaging bags, such as flexible pouches, formed by heat-sealing such biaxially stretched polyester films together, will be produced.
However, oriented films have poor heat sealing properties. In this regard, for example, Patent Literature 1 discloses a method of imparting heat sealing properties to a biaxially stretched polyester film by irradiating the surface of the film with short pulses of electromagnetic waves and modifying the surface.